This invention relates to an automatic focusing device suitable for use in an apparatus such as a video camera.
An automatic focusing device for use in an apparatus such as a video camera is known from the disclosure of, for example, Japanese Patent Publication No. 56-54610 published on Dec. 26, 1981.
The disclosed automatic focusing device comprises a light emitting element and a center-split light sensor including a pair of light-receiving elements. A beam of light emitted from the light-emitting element and reflected from a subject being photographed is received by the two-parts of the center-split light sensor. A confusing signal is obtained by detecting a displacement of a spot of the reflected light beam projecting onto the light sensor from its center based on a difference between the outputs of the two parts of the center-split light sensor.
In such an automatic focusing device, a motor is rotated by a controlled amount required for bringing the camera's lens system to the position of sharp focusing when such beam spot displacement is detected. The controlled amount of rotation of the motor is determined by the focusing signal which is variable according to a detected period of time T, that is, the time difference (t.sub.1 -t.sub.2) between times t.sub.1 and t.sub.2 at which integrated voltages V.sub.A and V.sub.B obtained by integrating output signals Sa and Sb of the two parts of the light-receiving elements, respectively attain the level of a threshold voltage (a reference voltage) V.sub.S. Therefore, it is possible that the time period T and the corresponding focusing signal obtained in one case are the same as those obtained in another case even though the amount of displacement of the beam spot from a center of the light sensor in the former case is different from that in the latter case. As a result, in one of the cases where the amount of beam spot displacement is smaller than that in the other case, the amount of focus adjustment by the focusing signal is excessively large, resulting in hunting. On the other hand, in the other defocusing case where the amount of beam spot displacement is larger than that in the one case, the amount of focus adjustment by the focusing signal is excessively small, resulting in a considerably slow response speed.